4.2. Post natal skull development in small African Pangolins
Samples obtained from Ibadan geo-location in (Fig. 3a) demonstrated more robust size and shape variations; being more disposed on the first axis of the PCA plot with size contributing a higher proportion of variations observed in comparison to similar samples from Kwara and Makurdi locations where shape variations factor overrode as they are better disposed on the second axis of the plot. Between the groups (Fig. 3b), the observed differences perhaps explain the input of environment on skull phenotypes development (Elliot, 2010; Benitez et al., 2020). Skull samples from Makurdi demonstrated the highest population variation in both size and shape factors compared to those from other geo-ecologies. Environmental prevailing temperatures, vegetation and diet variables as well as predator influences seem to favor samples from Ibadan eco-environments when compared to those from Guinea and Sudan savannah (Kwara and Makurdi) respectively. Large areas of overlap existed between the samples evaluated as shown in the MANOVA and ANOSIM analyses which failed to discriminate the species on geographical bias and was confirmed similar respectively; this is indicative of topographical area landmark similarity among population evaluated, such areas did not give any untoward developmental signal but follow phylogenetic trajectory compared to areas of non-overlap and is consistent with the report on similar extant species (Ferreira-Cardoso et al., 2020).
Skull structural flexibility potentials may have been exhibited by certain trait-variability patterns in these studied samples ontogeny which may be consequential in evolutionary trend. (Fig. 3b, Tables 1 and 2) confirmed the suggestion of Hendrikse et al. (2007) where subtle rostrum and dorsal skull deviations from midline (Fig. 4) were detectable and attributable to genetic input and lateralization in muscle load use or other functional demands (Urbanova et al., 2014). Maximum size and shape overlap observed (Fig. 3b) among sample populations from the geo-locations suggests high similarity this was further confirmed by an analysis of similarity (ANOSIM) and a multivariate analysis of variation (MANOVA) (Fig. 3c) on both skull views understudied. Rostrum shape overlaps between the rainforest (Ibadan) and Guinea savannah (Kwara) samples seem to progress in diversity as the species geographical space goes northwards to the Sudan savannah belt, they are however not been reported in the Sahel.
During craniofacial development, cranial neural crest cells’ migration to (frontal part of head) generate the facial skeleton (Le Douarin and Kalcheim, 1999) to become the mesenchyme of future face while the back of the skull is derived from a combination of neural crest-derived and mesodermal bones (Murphy et al., 2001a). Skull bones are derived from both the neural crest and the head mesoderm (Le Lievre 1980; Noden 1978; Klingenberg, 2010). Results from our study exposed some unexplained cranial asymmetries among the population investigated likely to be consequential to inconsistencies in cellular migrations; this is the most probable explanation to the observed unapparent skull asymmetries despite the absence of masticatory function in this species. Furthermore, the ecologic definition of arboreal living in species’ habitat substrates to their cranio-facial morphotypes with compensatory mechanisms in skull morphologic equilibrium as adaptations to their peculiar environment (Ritchsmeier & Deleon, 2009); though insignificant (F1539=3.4045, F882= 3.2665); for the skull views evaluated respectively demonstrated surreptitious fluctuating asymmetry (FA) (Tables 1 and 2); a measure of developmental instability (Klingenerg and McIntyre, 1998) in minute randomly distributed anomalies associated with environmental signals (Urbanova, 2014) on either (right and left) sides of skulls in the population evaluated in an otherwise bilateral development This is further substantiated by foramen magnum asymmetry assessments where size was an overriding factor (table 6) over shape.
Occurrences of FA in paired body structures have been attributed to ecologic, habitat, on-going metabolic disease conditions among population groups (Singh and Rosen, 2001). It is also known that a sustained unilateral body-side muscular load demand preference would directly modulate external observable morphology (Urbanova, 2014). Results from this study only confirmed a weak occurrence of FA (F=0.00034755). Foramen magnum development in the current study further predicated its importance in accurate forensic analyses, types of abnormalities encountered, proportions and interpretations in embryologic and evolutionary contexts among species as well as allometric trajectory pattern discriminations. Such information has no priory literature evidence in Pangolins. Salient neurological, developmentally consequential information hitherto not described could be inferred from the results obtained from EFA; the dorsal-most rim, right dorso-lateral, left dorso-lateral portions presented the discriminating areas of individual shape variations (Fig. 6, Table 6). Manoel et al. (2009) documented the possibility of cerebellar protrusion resultant upon volume reduction of the posterior fossa (pre and post natal); syringomelia and other neurological disorders sequel to foramen magnum dysmorphology manifested by the occurrence of dorsal notches as a result of developmental errors. Such structural malformations have been associated with domestication attempts and captive breeding of species (Dixon et al., 1997; Hewitt, 2011); this finding satisfies the 4th objective of this study. The results of the present investigation notwithstanding; is limited by an absence of fetal skulls thus prenatal studies will further confirm the onset and form of asymmetries in these pangolins.